The University of New Hampshire’s Center for Coastal & Ocean Mapping collected ~321,500 km2 of bathymetry and backscatter data of the U.S. Gulf of Alaska continental margin (Fig. 1a) using a 12-kHz multibeam echosounder in support of the U.S. Extended Continental Shelf (United Nations Law of the Sea Treaty) project (Gardner and Mayer, 2005). A previously unmapped small submarine fan, unofficially named Chatham fan (Figs. 1b and 1c), was identified that has buried a portion of the head of the much larger Baranof Fan (Stevenson and Embley, 1987; von Huene, 1989; LeVoir et al., 2011). Chatham fan either buried or diverted to the NW the upper reaches of Horizon Channel. The existing levee of upper Horizon Channel was subsequently incised by a remarkable meandering channel that trends along the strike of the levee, not down the steeper lee side of the levee slope. Although not directly related to Chatham fan, the meandering levee channel is related to processes that were contemporaneous with the deposition of Chatham fan.

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The University of New Hampshire’s Center for Coastal & Ocean Mapping collected ~321,500 km2 of bathymetry and backscatter data of the U.S. Gulf of Alaska continental margin (Fig. 1a) using a 12-kHz multibeam echosounder in support of the U.S. Extended Continental Shelf (United Nations Law of the Sea Treaty) project (Gardner and Mayer, 2005). A previously unmapped small submarine fan, unofficially named Chatham fan (Figs. 1b and 1c), was identified that has buried a portion of the head of the much larger Baranof Fan (Stevenson and Embley, 1987; von Huene, 1989; LeVoir et al., 2011). Chatham fan either buried or diverted to the NW the upper reaches of Horizon Channel. The existing levee of upper Horizon Channel was subsequently incised by a remarkable meandering channel that trends along the strike of the levee, not down the steeper lee side of the levee slope. Although not directly related to Chatham fan, the meandering levee channel is related to processes that were contemporaneous with the deposition of Chatham fan.

Description

Chatham fan is a 57 km long by 62 km wide deposit that formed on the uppermost slope directly west of Chatham Strait (Martin and Williams, 1924) on the glaciated west coast of Alaska (Fig. 1b). The fan stands ~500 m above the surface of Baranof Fan on the south. Chatham fan covers an area of ~3530 km2 and represents >885 km3 of sediment. The fan ranges in water depths from 1400 to 2800 m and is surrounded on all but the upslope side by incised distributary channels.

Distributary channels on Chatham fan vary in depth from less than 5 m to more than 50 m and spread out from at least two foci of deposition, best seen in the backscatter data (arrows on Fig. 1d). The distributary channels are clearly seen as higher backscatter fingers that radiate down the 1˚ to 3˚ slope of the fan surface and terminate in the deeply incised upper Horizon Channel. A 200 to 250 m high levee has developed on the northern side of Horizon Channel with a width that extends 250 km to the northwest. This side of the levee has slopes of ~0.2˚ whereas the slope of the strike of the levee has slopes of ~0.1˚ (Fig. 1c).

The upper 5 km of Baranof Fan channel thalweg has a slope of 2.5˚ whereas the remainder of the mapped portion of the channel thalweg has a slope of ~0.2˚. The channel thalweg is very flat with no evidence of landslide deposits on the channel floor (at the resolution of the 12-kHz multibeam bathymetry). The thalweg in the upper 84 km of Baranof Fan channel has a relative uniform width of 400 to 500 m but the thalweg width abruptly increases to 1.0 to 1.5 km at the southwest end of Chatham fan. The upper reach of Baranof Fan channel is linear in the uppermost section but becomes winding (1.25 sinuosity index) beginning at the southwest end of Chatham fan.

A large sinuous channel (sinuosity index 1.48) has incised the levee of Horizon Channel (Fig. 1c). The incision depth ranges from 100 to 200 m and the thalweg of this levee channel varies in width from 1 to 3 km along its course. The levee-channel floor is perched 100 to 200 m higher than the adjacent Baranof Channel floor (Fig. 1e). The meander channel has a levee of its own, rising as high as 100 m above the crest of the levee of Horizon Channel.

Interpretation

Chatham fan formed when a glacial tongue of Wisconsin age plowed a terminal moraine out of Chatham Strait (Manley and Kauffman, 2002), pushed it beyond the continental shelf and deposited it onto the upper portion of the much larger and older Horizon Fan. The superposition of Chatham fan onto the older Horizon Fan indicates the youthful age of Chatham fan. A similar glacial tongue of Wisconsin age extended across the shelf just north of Chatham fan but did not extend to the shelf break; consequently, little morainal material was pushed over the shelf break. However, it appears that sustained hyperpycnal flows (i.e., Mulder et al., 2003), generated by melting of the ice tongue, cascaded down the slope and eroded the meandering channel into the levee of Horizon Channel. The process might have been similar to jӧkulhlaup floods (Erlingsson, 1994; Maizels, 1997). Curiously, the runoff channel did not erode down the steeper lee side of the levee perhaps because of the rapidly formed meander-channel levee that blocked the steeper path for the channel. However, a crevasse splay does exist in the meander-channel levee (Fig. 1c) suggesting the flow eroded through the levee and flowed down the steeper gradient at least once. It is unclear, but appears reasonable that Chatham fan buried a pre-existing Horizon Channel. In addition, the sinuous nature of the meander channel at 55˚30’N appears at odds with one of the conclusions of Peakall et al. (2012) that channel sinuosity decreases with increasing latitude.

GARDNER, J.V. and MAYER, L.A., 2005. U.S. Law of the Sea cruise to map the foot of the slope and 2500-m isobaths of the Gulf of Alaska continental margin, cruises KM0514-1 and KM0514-2. CCOM/JHC Administrative Report 05-1, 11p, http://ccom.unh.edu/sites/default/files/publications/ Gardner_05_cruise_report_KM0514.pdf.